This invention relates to gas turbine engines with overhung shafts.
The typical gas turbine engine includes a compressor wheel and a turbine wheel mounted on a shaft that is journaled to a housing by two bearings. Ideally, the bearings are located at opposite ends of the shaft. In some engines, one bearing is located at one end of the shaft and the second bearing is located at or near the middle of the length of the shaft. The portion of the shaft extending from the middle bearing to the end of the shaft without a bearing is referred to as the overhung shaft portion. In engines having overhung shaft portions, it is not uncommon to find both the compressor wheel and the turbine wheel mounted on this overhung portion.
A disadvantage to this engine configuration is that the rotating component on the overhung shaft portion farthest from the middle bearing has a tendency to droop or sag due to its own weight. For a variety of reasons, this component is usually the turbine wheel and because of this droop, the turbine blades can rub against the turbine shroud circumscribing the blades. This drooping is illustrated in FIG. 3 which shows a turbine wheel 100 circumscribed by a shroud 102 both of which are concentric about an axial extending centerline 103. Using conventional gas turbine nomenclature, the top 104 of the engine is referred to as the twelve o""clock position and the bottom 106 the six o""clock position. Because of the droop, the clearance between the turbine wheel 100 and shroud 102 is greater at the twelve o""clock position and less at the six o""clock position. Thus, the Applicants have observed rubs between the wheel and shroud at the six o""clock position. Another disadvantage is that as the wheel droops, the clearance between the blades and shroud at the twelve o""clock position increases, thus reducing engine performance.
One proposed solution is to machine a groove into the turbine shroud extending from the five o""clock position to the seven o""clock position. While such a groove avoids the rub, it does not improve the clearance at the twelve o""clock position. Further, the Applicants have found that the machining of this groove is very expensive.
Accordingly, there is a need for a new and useful wheel and shroud arrangement that accounts for rotor droop associated with overhung shaft portions.
An object of the present invention is to provide a novel arrangement of shroud and wheel that accounts for rotor droop associated with overhung shaft portions.
The present invention meets this objective by providing a gas turbine engine having a housing and an axially extending shaft journaled to the housing by two axially spaced apart bearings. The bearings are located along the shaft to define an overhung shaft portion. A rotating component such as a turbine wheel or compressor wheel is mounted on the overhung shaft portion and is concentric about the centerline of the engine. An annular shroud that is part of the housing is disposed about the rotating component, the annular shroud being concentric about a centerline radially offset from the engine centerline.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.